that would be pushing to the maximum. It falls within the range, but is close to maximum, and I don't believe in pushing to the max if you desire a stiff floor.

PT itself isn't listed but if dried it should be equivalent to non-treated.

As for beams, NO, those double 2x8's with only 3 piers in 20 feet will not do the job. Off the top of my head you'd need a pier/post every 4 feet; six per side. That is if this is for a cabin. That's going by the sizes and numbers used in John's Little House plans.

« Last Edit: January 31, 2009, 02:54:14 PM by MountainDon »

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Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

I figure if I run 3 rows of beams 20' long then I can run the 16' floor joist across them. Since I am going with 3 beams I assume I could get away with 2x8's at 16" OC?

If I use 6x6's for the posts under the 3 beams is there an easier way to attach them rather than notching them out? Is there a type of connector to fasten the 4x8 (two doubled up 2x8's) to the top of the 6x6's?

It wasn't clear from your original post that there would be three beams supporting the floor joists.

Looking at the joist question, if there are three beams under them, 2x8 will work. You mentioned 2x10 in the first post and those would be pushing the max on two beams.

However, the addition of a central beam does very little to lighten the loads carried by the two outside beams. Those beams carry the bulk of the weight of the cabin, the side walls and the ceiling and roof loads (assuming you have a gable roof with the ridge running in the long direction.

To use a 4x8 or doubled 2x8's there you need more posts. You can use 4x4's for posts. Place a 1/2 plywood spacer between the two 2x8's and the width will match that of a 4x4. You can them use standard Simpson post to beam brackets. That would be better than notching out the 6x6's.

Not only do you need more posts to prevent flex in the 4x8 beams, but you may need more posts and footings to spread the weight of the building and contents out over enough ground.

If you want fewer posts because of difficulty in digging the ground you could go to 7 foot spacing with a 6x12, maybe a 6x10 depending on the load/weight of the building and contents. This is a detailed calculation that is beyond the scope of this forum. You would be safe with a 6x12 (three 2x's with two 1/2" spacer layers). I used a four layer sandwich of 2x material, no 1/2" spacer material, on my cabin.

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Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

I would like get away with the least amount of holes as possible. I just priced out the 2X12's for floor joist and they are not that much more so I think I will span the 16 foot width with 2x12's. Would this allow me to get rid of the middle beam??? In other words if I go 2x12 floor joists 16" OC can I go with two outside beams only?

I see the hint but I'm going to push the envelope a little hoping to show how some of this works in case anyone wants to try it, I hope I'm not stepping in it .This floor looked to me like it might be marginal bouncy wise so I put some numbers to it real quick. Not knowing where you are I used my snow loads on the roof to size the girder... totally bogus but it told me how the shop floor would perform in my back yard.I used 20 psf snow and 10 psf roof dead load, no ceiling, 40 psf floor live, 10 psf floor frame... that is real quick and dirty, don't build by that it is for example only.

I assumed the girders would span between posts about 6 feet and would aupport half the width of the floor and half the width of the roof+ overhang. for the floor then it would be 6' long, 8'wide and 50 psf =2400 lbs. for the roof 6' long, 8+1(overhang)x 30 psf =1620 lbs... 2400+1620=4020 lbs on the girder

So I had a rough design, Here's what I really wanted to check, the vibration frequency, the calculator is here;http://windyhilllogworks.com/Calcs/floorfreq.htm This is a screenshot;The joist is marginally acceptable bouncewise at a typical load, the minimal girder I came up with makes the floor a real dog. i went to a 6x10 girder and was still unacceptable in overall floor performance. Then I started thinking about my shop, 3 engines, a stack of wood and the tools. As I added that weight it doesn't collapse but that middle girder starts looking like a real good idea.

This is a shop, so you can apply that however far you want. I noticed there had been several bouncy floor comments so I'm sort of using this example to show how to get an idea about how bouncy a floor will be.

This is a shot of the first part of the floor frequency check. I'm checking the vibration frequency of the joist. The red arrow points to where I've entered the actual dimensions of a 2x12 floor joist;

The load on a joist is an area halfway to each ajoining joist in width and to the center of the bearing girder in lengthSo 1.33 feet wide X 15.6 feet long x an average of what most floors systems actually weigh in service 7 lbs per square foot= this comes up around 145 lbs bearing on the joist in average service conditions. I know the average modulus of elasticity for #2 SYPine is 1.6 Million psi from tables, that is its stifness, or to think about it another way, that is the average tension a #2 pine guitar string is tuned to. To continue the analogy, from bridge to fret is 15.6 feet, tuned to 1.6 MPSI , the string is shaped like a 2x12 and weighs 146 lbs. When struck the joist rings at a natural frequency of about 17 cycles per second. Researchers have done studies that show that we consider a floor annoying at below about 15 hz. This is getting close. The frequency of the girder combined with this it will likely feel bouncy.

If you want to continue down that path of thinking, put 300 lbs on the joist and the frequency of the joist drops lower into the annoying zone. That's more like my shop loading, I seem to accumulate heavy stuff in there.

Conventional building technique places the beams under the long walls as that is the usual direction the roof ridge runs (gable roof). THat directs the roof loads to the beams in the most direct and strongest way.

Check that AWC calculator link above to check sizing on the floor joists. You can adjust the input data for heavier than norma; (40 lb/sq ft) loads if you have lots of heavy equipment.

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Just because something has been done and has not failed, doesn’t mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Thanks John,Yup you guessed it, I went existential over the whole Fv readjustment and just never went back to fix it. Now they've added Emin... another rainy day job.

I had not seen your book of design values, Thanks! I wish they had added Fv to their table but the size factor and load durations are already done, that's a big help.

I've been using this one for design values, it has shear values and the new E-min value for columns but doesn't have the design value adjustments in table form by size as is done in yours.http://www.awc.org/pdf/2005-NDS-Supplement.pdf

If you right click on the page on my calcs and then click "view source" the javascript is all there if you want to check it or modify. If they used examples from real life like this when they taught math, I wonder if math would stick better. If every high school kid could size a beam I wonder if we would have a situation where cities can demand every plan needs a design professional .

Back to the problem at hand, there are girder and header tables in the IRC codebook , chapter 5 table R502.5(2) lists a double 2x10 good under a 20' wide building supporting one floor only at a girder post max spacing of 7'. In the tables that 2-2x10 girder is good at the perimeter as well. a 2x6 SYP joist is good for 9'9" in the tables, So that is the minimums. That floor combination vibrates at 14.1 hz so it performs worse than full span 2x12's that vibrated at about 14.7 hz.

This is where you can use the vibration program to see what combination of parts gives the best results for the parts invested. I interchanged different combinations, almost always "deeper is cheaper", going a size deeper usually has more effect on performance than adding another shallower width ply.

I was curious so ran some different combinations to see how the floor would stiffen up;A code minimum floor rings at between 14.1-14.7 Hz. I think a tuning fork rings at 440 hz so this is a real dull thud.The floor vibrates at 19.7 hz when going to 2x8 SYPine joists and double 2x10 girderGo to a double 2x12 girder with 2x8 joists and it vibrates at 20.7 hzWith a double 2x10 girder and 2x10 joists it vibrates at 25 hzWith a double 2x12 girder and 2x10 joists it vibrates at 27.9 hzWith 2x12 joists and girder it vibrates at 34.25 HzBumping the last option from #2 grade to Dense #1 raises the fequency to 36.33 Hz

Beyond the code and AWC minimums it is purely judgement as to how you want the floor to feel. My own shop building started as a flower drying barn and I framed it with 2x8's 12' spans on double 2x12 girders. it was fine till I brought in 3 engines a '35 Dodge truck, bundles of lumber, tools... I now need to crawl under that deadfall trap and insert midspan girders.

I went back to see how strong the 2x10' joists on 2x12 girders combination worked out to be, it was good for 7500 lbs overall on the girder and 1700 lbs per joist or 125 lbs per square foot... OK, guess I like a brick house for a shop floor

I went inside yesterday when the snow started and defected through some locust lumber just to kind of clean up and finish out the day. Boy I should have worn a mask. Both bloody nostrils and head running like a seive, thats some nasty stuff in an enclosed area.

Don_P,Thanks a lot for your help. I went to the IRC codebook , chapter 5 table R502.5(2) and it's very informative. I think these tables will give me a lot to work with. Now I just need to settle on what I want to use for posts! Is there any type of table or a rule that explains the size and thickness of footings needed under a sonotube or 6x6 post?